The present invention relates to a circuit for generating an output voltage which is proportional to temperature with a required gradient.
Such circuits exist which rely on the principle that the difference in the base emitter voltage of two bipolar transistors with differing areas, if appropriately connected, can result in a current which has a positive temperature coefficient, that is a current which varies linearly with temperature such that as the temperature increases the current increases. This current, referred to herein as Iptat, can be used to generate a voltage proportional to absolute temperature, Vptat, when supplied across a resistor.
Although this principle is sound, a number of difficulties exist in converting this principle to practical applications. One such difficulty is that, in existing circuits, the voltage which is generated remains positive even when the temperature undergoes negative variations, that is temperature variations below 0xc2x0 C. This means it is not possible to generate a Vptat which directly maps the temperature.
It is an aim of the present invention to provide a circuit which will allow the voltage proportional to temperature to vary negatively with negative temperatures.
The present invention provides a circuit for generating an output voltage proportional to temperature with a required gradient, the circuit comprising: a first stage arranged to generate a first voltage which is proportional to temperature with a predetermined gradient but which has a positive value when the temperature falls below zero; and a second stage connected to the first stage and comprising a differential amplifier having a first input connected to receive the first voltage and a second input connected to receive a feedback voltage which is derived from an output signal of the differential amplifier via an offset circuit which introduces an offset voltage such that the output signal of the differential amplifier provides at an output node said output voltage which has a negative variation with negative temperatures.